Signal travel time flow meter

ABSTRACT

A method for determining a flow speed of a liquid in a fluid conduit is provided. During a signal-generating phase, an impulse signal is applied to a first ultrasonic transducer. A response signal is then received at a second ultrasonic transducer. A measuring signal is later derived from the response signal, wherein the derivation comprises reversing a signal portion with respect to time. During a measurement phase, a liquid moves with respect to the fluid conduit. The measuring signal is then applied to one of the two transducers and a response signal of the measuring signal is measured at the other transducer. A flow speed is derived from the response signal of the measuring signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. § 120 ofInternational Application No. PCT/IB2015/055724, filed Jul. 29, 2015,which claims priority to International Application No.PCT/IB2014/063502, filed Jul. 29, 2014, the contents of each are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The current application relates to flow meters, and in particular toultrasound travel time flow meters.

BACKGROUND

Various types of flow meters are currently in use for measuring a volumeflow of a fluid, such as a liquid or a gas, through a pipe. Ultrasonicflow meters are either Doppler flow meters, which make use of theacoustic Doppler effect, or travel time flow meters, sometimes alsocalled transmission flow meters, which make use of a propagation timedifference caused by the relative motion of source and medium. Thetravel time is also referred to as time of flight or transit time.

An ultrasonic travel time flow meter evaluates the difference ofpropagation time of ultrasonic pulses propagating in and against flowdirection. Ultrasonic flow meters are provided as in-line flow meters,also known as intrusive or wetted flow meters, or as clamp-on flowmeters, also known as non-intrusive flow meters. Other forms of flowmeters include Venturi channels, overflow sills, radar flow meters,Coriolis flow meters, differential pressure flow meters, magneticinductive flow meters, and other types of flow meters.

When there are irregular flow profiles or open channels, more than onepropagation path may be necessary to determine the average flow speed.Among others, multipath procedures are described in hydrometry standardssuch as IEC 41 or EN ISO 6416. As a further application, ultrasonic flowmeters are also used to measure flow profiles, for example with anacoustic Doppler current profiler (ADCP). The ADCP is also suitable formeasuring water velocity and discharge in rivers and open waters.

It is an object of the present specification to provide an improvedtransit time flow meter and a corresponding method for measuring anaverage flow speed or a flow profile of a fluid in general, and inparticular for liquids such as water, oil or for gases.

SUMMARY OF INVENTION

In a flow measurement device according to the present specification,sound transducers, e.g. in the form of piezoelectric elements, alsoknown as piezoelectric transducers, are used to generate and to receivea test signal and a measuring signal.

Alternative sound transmitters comprise lasers that excite a metalmembrane to vibrations, or simple loudspeakers. One can also producepressure waves in other ways. The receiver side can also be representedby other means that are different from piezoelectric transducers, butdetect ultrasonic waves.

Although the term “piezoelectric transducer” is used often in thepresent description, it stands also for other sound wave transducersthat produce or detect ultrasonic waves.

A measuring signal according to the present specification can bemodelled by a matched filter. If a sharply peaked impulse is used as aprobe or test signal, the received signal at the transducer is theimpulse response of a conduit or channel of the fluid. According to thepresent application, an inverted version of the impulse response withrespect to time is sent back through the same channel as a measuringsignal, either in the reverse direction or in the same direction. Thisresults in a signal with a peak at the origin, where the original sourcewas, or in a signal with a peak at the original receiver, respectively.

The inversion with respect to time can be achieved in several ways. Ifanalogue means are used for recording the response signal, one couldplay the recorded response signal in a reverse mode. If digital meansare used for recording samples of the response signal, then the order ofthe recorded samples is reversed in order to obtain the inverted signal.This can be achieved by inverting the values of the time stamps of eachrecorded sample, by multiplying the respective time value with (−1). Ifplayed according to an ascending order of the time stamp values, therecorded samples are played in a reverse order. In other words, theinverted response signal is the recorded response signal, but playedbackwards.

An ultrasonic flow meter according to the present specification providesa focusing property by using the above mentioned inverted signal, or asimilarly shaped signal, for an ultrasonic flow meter to form a responsesignal, which is both concentrated in space and time. This in turn leadsto a higher amplitude at a receiving piezoelectric element and a bettersignal to noise ratio.

With an ultrasonic flow meter according to the present specification,focusing can be obtained under very general conditions. For example, afocusing property is obtained even when only one ultrasound transmitteris excited and even when the inverted signal is reduced to a signal thatis only coarsely digitized in the amplitude range, if the timeresolution of the inverted signal is sufficient. Furthermore, a flowmeter according to the present specification can be used with clamp-ontransducers, which are easy to position on a pipe and do not requiremodifications of the pipe.

In a flow measurement method according to the present specification, abit resolution with respect to an amplitude of the measurement signalcan be adjusted. In particular, the bit-resolution can be adjusted toobtain a high amplitude of a response signal.

According to one embodiment, the bit resolution is increased forincreasing an amplitude of a response signal to the measuring signal. Inone embodiment, the bit resolution is increased in pre-determined steps,the bit resolution which produces the response signal with the highestamplitude is selected and a corresponding representation of ameasurement signal is stored in computer memory.

According to another embodiment, the bit resolution is decreased forincreasing an amplitude of a response signal to the measuring signal. Inone embodiment, the bit resolution is decreased in pre-determined steps,the bit resolution which produces the response signal with the highestamplitude is selected and a corresponding representation of ameasurement signal is stored in computer memory.

In particular, the bit resolution may be a low bit resolution, such as aresolution that is stored in one digit or in two digits, in particularin one or two binary digits. According to other embodiments, the low bitresolution comprises at least a 1 bit resolution and at most a 64 bitresolution.

According to a further embodiment, the first response signal isprocessed for determining or deriving a change in the wall thickness ofthe conduit or for determining or deriving material characteristics ofthe conduit wall by determining longitudinal and transversal sound wavecharacteristics. For example, the transverse and longitudinal wavescharacteristics may be derived from corresponding portions of thereceiving or response signal, which corresponds to different times ofarrival of the acoustic waves.

According to this embodiment, the same response signal is used for thedetermination of the flow speed and for the detection of theabovementioned properties. Thereby it is no longer necessary to use aseparate signal or a separate arrangement to detect effects such ascontaminations and material faults, although a separate signal or aseparate arrangement may be used. Furthermore, the derived channelproperties can be used to obtain a more accurate estimate of the flowspeed.

In an ultrasonic flow meter according to the present specification,technical features that ensure a good coupling and directionality ofclamp-on transducers and to reduce scattering may not be necessary or,on the contrary, it may even improve the focusing. In order to providean increased scattering, a coupling material may be selected that isadapted to a refractive index of the liquid or transducers andtransducer couplings may be used which provide more shear waves.

Preferentially, the frequency of sound waves that are used in a flowmeter according to the specification is between >20 kHz and 2 MHz, whichcorresponds to an oscillation period of 0.5 microseconds (μs) but it mayeven be as high as 800 MHz. In many cases, ultrasonic flow metersoperate far above the hearing threshold with frequencies of severalhundred kHz or higher. The frequency of transit time ultrasonic flowmeters is typically in the kHz or in the MHz range.

According to one aspect, the current specification discloses a computerimplemented method for determining a flow speed of a fluid in a fluidconduit or channel using a transmission time ultrasonic flow meter. Inparticular, the method can be used for a pipe or tube, but also for openchannel applications, such as applications for drainage or irrigationchannels. In a preferred embodiment, “computer implemented” refers to anexecution on small scale electronic components such as microprocessors,ASICs, FPGAs and the like, which can be used in portable or in compactstationary digital signal processing devices, which are generally of asmaller size than workstations or mainframe computers and which can beplaced at a required location along a fluid pipe.

In the following, the terms “channel”, “conduit”, “passage”, “pipe”,etc. are used as synonyms. The subject matter of the application can beapplied to all types of conduits for fluids independent of theirrespective shape and independent of whether they are open or closed orfully filled or partially filled. The subject matter of the applicationcan also be applied to all types of fluids or gases, whether they aregases or liquids, or a mixture of both.

During a measuring signal generating phase, the fluid conduit isprovided with a fluid at a predetermined velocity with respect to thefluid conduit, especially with a fluid that is essentially at rest withrespect to the fluid conduit. The measuring signal is generated from aresponse signal, which the transmission channel generates in response toan initially applied impulse signal.

An impulse signal is applied to a first ultrasonic transducer, such aspiezoelectric transducer, wherein an impulse signal refers to a signal,which has a signal energy that is concentrated over a short period oftime in particular In a specific embodiment, the impulse signal extendsover only a few oscillation periods of a carrier, such as 10-20oscillations periods or less. In particular, an envelope of the impulsesignal may have a rectangular shape, but other shapes are possible aswell. For example, the impulse signal may correspond to a one-time peakor a single impulse, a short rectangular burst or to any other signalshape, such as a saw-tooth shape, a rectangular wave, a chirp, a sinewave or a pre-determined noise burst, such as a white noise or a pinknoise, which is also known as 1/f noise. The method works with almostany signal shape of the impulse signal.

The signal generating phase does not need to be repeated for eachmeasurement. For example, it may be carried out before a firstmeasurement and at later times when the conditions in the fluid conduitchange, for example due to sediments, corrosions and thermal stress.

Sometimes, the term “calibration phase” is used when referring to themeasuring signal generating phase. This is not entirely correct. Forflow meters, it is typical that the flowmeter is placed at a calibrationrig where the measured values and the target values for flow rates arecompared. The linking factor between these two values is calledcalibration factor and it incorporates hardware and software errors ofthe flow measurement which cannot be specified. For the subject matterof the application, it is more appropriate to discern between themeasuring signal generating phase and the calibration phase. Themeasuring signal generating phase provides a measuring signal that—whenused—delivers a relatively sharp peak in the response signal to themeasuring signal, while the calibration phase provides a flow meter thatprovides a precise flow rate measurement.

The following steps of the method according to the specification:

-   -   providing an impulse signal to a first ultrasonic transducer,        the first ultrasonic transducer being located at the fluid        conduit at a first location,    -   providing a response signal of the impulse signal at a second        ultrasonic transducer, the second ultrasonic transducer being        located at the fluid conduit at a second location,    -   deriving a measuring signal from the response signal, the        derivation of the measuring signal comprising selecting a signal        portion of the response signal or of a signal derived therefrom        and reversing the signal portion with respect to time, can be        provided by applying and measuring real actual signals to a real        conduit. It turned out that the steps of providing a response        signal of the impulse signal at a second ultrasonic transducer        and of deriving a measuring signal can be obtained by a        numerical or analog simulation, once the impulse signal is        provided as a digital or analog signal. Finite element software        can be used for this purpose.

The piezoelectric transducers are located at the fluid conduit. Inparticular, they can be located at respectively mounted to the fluidconduit. The first piezoelectric transducer is located at respectivelymounted to a perimeter of the fluid conduit at a first location. In oneparticular embodiment, it is clamped onto the perimeter of the fluidconduit. A response signal of the impulse signal is received at a secondpiezoelectric transducer.

The second ultrasonic transducer, such as piezoelectric transducer islocated at respectively mounted to the fluid conduit at a secondlocation, which is offset along a longitudinal direction of the fluidconduit with respect to the first location and along a cross-sectionwhich goes through the center of the conduit axis, wherein thelongitudinal direction corresponds to a direction of average flowthrough the channel. The fluid conduit can be completely filled with thefluid if reflections at the fluid surface and other effects are notwanted.

A measuring signal is derived from the response signal, which is aresponse of the transmission channel to an initial impulse signal, withanalog means or also digitally. The derivation of the measuring signalcomprises selecting a signal portion of the response signal or of asignal derived therefrom and reversing the signal portion with respectto time, and it may comprise the step of storing measuring signal, e.g.in its digitized form in a computer readable memory for later use.Herein, different sequences of the method steps are possible. Forexample, the signal may be inverted with respect to time after storingit.

During a measurement phase, in which the fluid moves with respect to thefluid conduit according to external conditions such as pressure,gravity, inclination of the pipe etc., the measuring signal is appliedto one of the first and the second ultrasonic transducers, such aspiezoelectric transducers. More particular, an electric signal, whichcan be derived from a stored measuring signal, can be applied to thetransducer.

A first response signal of the measuring signal is measured at the otherultrasonic transducer, such as piezoelectric transducer, a flow speed ofthe fluid is derived from at least the first response signal. Inparticular this comprises measuring a downstream or upstream time offlight. An estimate of the velocity may be obtained by comparing themeasured time of flight with a time of flight under calibration takinginto account the velocity of sound under the current conditions, forexample by measuring a temperature of the fluid. In further steps, avolumetric flow or a mass flow may be derived from the flow speed orfrom a flow speed profile.

In order to obtain a more accurate estimate, measurements may be carriedout in both directions, from the first to the second ultrasonictransducer, such as piezoelectric transducer and in reverse direction.In particular, this allows to eliminate the speed of sound in a time offlight measurement or it can provide a reliable estimate of the currentspeed of sound.

A flow measurement according to the present specification can be used into arrangements with only two transducers and also in multi-transducerarrangements, such as the arrangements of FIGS. 43 and 44 or thearrangement of FIGS. 4 and 5. In particular, the flow measurement can beobtained by a pair of transducers of a multi-transducer arrangement,which are arranged opposite to each other. The pair of transducers maybe arranged in a plane through a central axis of the conduit, as shownin FIG. 43 or they can be arranged in a plane that is offset withrespect the central axis of the conduit, as shown in FIG. 44. Thearrangement of FIG. 44 can be used to determine the fluid velocity in afluid layer at a predetermined distance to the central axis.

Accordingly, the steps of applying the measuring signal and measuringthe response signal are repeated in the reverse direction. In otherwords, the previous receiver is used as a sender and the previous senderis used as receiver and a signal is sent from the respective otherultrasonic transducer, such as piezoelectric transducer to therespective first one of the two transducers in order to obtain a secondresponse signal. A flow speed of the fluid is derived from the firstresponse signal and the second response signal. In particular, thederivation comprises deriving a downstream and an upstream time offlight.

Although one can send a measuring signal from one ultrasonic transducer,such as piezoelectric transducer to another ultrasonic transducer, suchas piezoelectric transducer, it is also beneficial to execute thisforward and reverse when a velocity or flow measurement is done.

In other words, the procedure can be done the following way.

Forward Direction:

-   -   Sending an impulse signal from a first ultrasonic transducer to        a second ultrasonic transducer,    -   Receiving a response signal of the impulse signal at the second        ultrasonic transducer,    -   Inverting the received response signal at the second ultrasonic        transducer with respect to time, thereby obtaining a measuring        signal,    -   Sending the measuring signal from the first ultrasonic        transducer to the second ultrasonic transducer,    -   Receiving a response signal of the measuring signal at the        second ultrasonic transducer.

Reverse Direction:

-   -   Sending an impulse signal from the second ultrasonic transducer        to the first ultrasonic transducer, such as piezoelectric        transducer,    -   Receiving a response signal of the impulse signal at the first        ultrasonic transducer,    -   Inverting the received response signal of the impulse signal        from the first ultrasonic transducer with respect to time,        thereby obtaining a measuring signal,    -   Sending the measuring signal from the second ultrasonic        transducer to the first ultrasonic transducer,    -   Receiving a response signal to the measuring signal at the first        ultrasonic transducer,    -   Measuring the time difference between the received response        signals at the second ultrasonic transducer and the first        ultrasonic transducer. This time difference is proportional to        the velocity of flow between the two ultrasonic transducers,        such as piezoelectric transducers.

Please note that the measuring signal for the forward direction can bedifferent from the measuring signal for the backward direction. Themeasuring signal has usually a unique shape for each direction ofpropagation, although for simple configurations identical measuringsignals can be used.

Throughout the application, the term “computer” is often used. Althougha computer includes devices such as a laptop or a desktop computer, thesignal transmission and receiving can also be done by microcontrollers,ACID's, FPGA's, etc.

Furthermore, a thought connection line between the transducers may begeometrically offset with respect to a center of the fluid conduit inorder to obtain a flow speed in a predetermined layer and there may bemore than one pair of transducers. Furthermore, the measuring signal maybe provided by more than one transducer and/or the response signal tothe measuring signal may be measured by more than one transducer.

According to a simple embodiment, an average measuring signal isgenerated by a linear superposition of the response signals from themultitude of receiving transducers and the abovementioned signalprocessing steps are performed on the average response signal to obtaina measuring signal.

According to yet another embodiment, there is an equal number, say N, ofsending and receiving transducers, wherein the relative placements ofthe sending transducers are equal to the relative placements of thereceiving transducers. The N received response signals are thenprocessed individually according to the abovementioned signal processingsteps to obtain N individual measuring signals.

These N transducers are typically arranged e.g. as clamp-on transducers,insertion or internal mount transducers. By way of example, FIG. 43shows an arrangement with 8 clamp on transducers and FIG. 44 shows anarrangement with 8 insertion transducers. The 8 transducers of FIG. 43are arranged in four respective planes, which are going through the axiscenter of the conduit.

The 8 insertion transducers of FIG. 44 are arranged in four parallelplanes.

The connection lines between the transducers show an operation mode ofthe transducers. In the operation mode of FIG. 43, signals are sent froma first transducer to a second transducer which is opposite to the firsttransducer with respect to a centre point on the centre axis of thewater duct.

In the operation mode of FIG. 44, signals are sent from a firsttransducer to a second transducer with respect to a centre point, whichis located at the centre of the respective rectangular arrangement andin one of the four parallel planes.

According to one embodiment the signal portion of the response signalthat is used to derive the measuring signal comprises a first portionaround a maximum amplitude of the response signal and a trailing signalportion, the trailing signal portion extending in time behind thearrival time of the maximum amplitude. The trailing portion providessignals from further reflections apart from the signals in the vicinityof the direct signal and can contribute to a better focusing.

In order to obtain an improved generated measuring signal, the steps ofapplying an impulse signal and receiving a corresponding response signalmay not only be done once but they can be repeated multiple times, atleast two times. Thereby, a plurality of response signals is obtained. Ameasuring signal is then derived from an average of the receivedresponse signals.

In one embodiment, the measurements are repeated multiple times but withthe ultrasound signal traveling in one direction only. In anotherembodiment, the measurements are repeated multiple times, the ultrasoundsignal travelling in both directions. In yet another embodiment, themeasurements are repeated multiple times in both directions and separateaverages are derived for both directions.

According to a further embodiment, the derivation of a measuring signalform one or more received response signals comprises determining anenvelope of the response signal or of a signal derived therefrom. Anamplitude modulated oscillating signal is provided which is amplitudemodulated according to the envelope. Using an envelope instead ofsamples, or in addition to it, may provide benefits in terms of storagespace and computation speed.

In particular, the modulation amplitude may have the shape of thedetermined envelope for the measuring signal or for a portion of it. Anoscillation frequency of a carrier oscillation is at least 20 kHz.According to further embodiments the frequency is at least 100 kHz, atleast 500 kHz, or at least 1 MHz. The choice of frequency affects thescattering process and a higher frequency may provide a finer-grainedsampling of a conduit wall, which may in turn allow a more preciseshaping of the ultrasound signal.

According to further embodiments, the response signal or a signalderived therefrom is digitized with respect to amplitude, and especiallywith a resolution between 1 and 8 bit. The present specification showsthat even a coarse digitization with respect to amplitude may lead to asufficient focusing of the ultrasound signal. Using a low resolutionsaves computing time and memory space, while higher resolutions do notnecessarily provide a more precise measurement result of the fluid flowrate through the conduit. It has also turned out that increasing ordecreasing the resolution of the response signal or the measuring signalcan help to improve the signal-to-noise-ratio and the precision of thetime measurement. Reducing the resolution results in a sharper or morecharacteristic peak in the response to the measuring signal. This meansthat if there is high SNR, one could decrease the resolution of themeasuring signal or the response signal to the measuring signal insteadof increasing the transmitting power of the measuring signal.

According to a further aspect the present specification, some methodsfor determining a flow speed of a fluid in a fluid conduit or pipe mayuse an amplitude modulated measuring signal or an amplitude modulatedresponse signal of a transmission time ultrasonic flow meter. Thismethod does not necessarily involve a signal generating phase step,although a one-time signal generating phase step may be used to obtain ameasuring signal. For example, the method may rely on a pre-generatedmeasuring signal at a factory site, wherein the measuring signal isgenerated as an with respect to time inverted receive signal of oneultrasonic transducer, such as piezoelectric transducer that hasreceived a series of oscillations sent out by another ultrasonictransducer, such as piezoelectric transducer.

In a first step, the fluid conduit is provided with the fluid, whichmoves with respect to the fluid conduit according to external conditionssuch as pressure, gravity, inclination of the pipe etc.

A first piezoelectric transducer is provided at a first location of thefluid conduit. A second ultrasonic transducer, such as piezoelectrictransducer at is provided at a second location of the fluid conduit. Thesecond location is offset along a longitudinal direction of the fluidconduit with respect to the first location, the longitudinal directioncorresponds to a fluid flow direction of the fluid channel.

A measuring signal is provided and applied to the first or to the secondultrasonic transducers, such as piezoelectric transducers. Moreparticular, an electric signal which is derived from an amplitudemodulated signal that can be sent to the transducer.

A first response signal of the measuring signal is measured at the otherultrasonic transducer, such as piezoelectric transducer, and a flowspeed of the fluid is derived from the first response signal. Inparticular, this comprises deriving a downstream or an upstream time offlight.

Similar to the abovementioned method, a higher precision may be achievedby repeating the measurement in the reverse direction to obtain adownstream and an upstream time of flight. As shown in FIGS. 43 and 44,N pairs of transducers can be utilized, for example for obtaining a moreaccurate estimate of the average flow or for obtaining an estimate ofthe flow in a plane at a predetermined distance from the central axis ofthe liquid duct.

In particular, the steps of applying the measuring signal and measuringthe response signal are repeated in the reverse direction to obtain asecond response signal, and a flow speed of the fluid is derived fromthe first response signal and the second response signal, wherein thederivation comprises deriving a downstream and an upstream time offlight.

These steps are very similar to the method steps as described above,with the difference that measurements are done without adjusting thedevice before each measurement.

The following features apply to both methods, with or without signalgeneration phase before each measurement.

According to a further embodiment, an amplitude of the measuring signalor an amplitude of the response signal can increase to a maximumamplitude over a predetermined number of oscillations, e.g. five or moreoscillation periods of the carrier signal. When the amplitude increasesover a period of time, an inertia effect of a reaction time of theultrasonic transducers, such as piezoelectric transducers on themeasurement can be reduced.

In one particular embodiment, the measuring signal or the responsesignal increases exponentially to a maximum amplitude over at least fiveoscillation periods of the carrier signal.

According to a further embodiment, the measuring signal comprises aleading portion, the leading portion extending in time over a number ofhalf-widths of a signal maximum of the measuring signal, and the leadingportion preceding at least one half-width region of the signal maximumin time.

According to yet another embodiment, the measuring signal comprises aleading portion. The leading portion is derived from a trailing portionof a received signal, which succeeds a signal maximum of the receivedsignal with respect to time. The leading portion extends over at leastthree times the half-width around the signal maximum of the receivedsignal.

According to further embodiments, the leading portion comprises at least10% or at least 50% of a signal energy of the measuring signal.

A signal energy E of a signal s(t) in a time interval may be defined interms of the expression E=∫_(T1) ^(T2)dt|s(t)|² or its discrete versionE=Σ_(i=−m) ^(n)|s(i)|², wherein the time interval is given by [T1, T2]or [−m*Δt, n*Δt], respectively.

The leading portion of the measuring signal may contribute significantlyto the production of a signal which is peaked in space and time.

In some specific embodiments, the measuring signal or the responsesignal can be provided by an amplitude-modulated oscillating signal,which is digitized with respect to amplitude, e.g. with a resolutionbetween 1 and 8 bit. This may provide benefits in terms of computationvelocity and memory space and can even lead to an increased signal peak.

According to a further embodiment, the measuring signal that is appliedto a transducer can comprise an oscillating signal that is modulatedaccording to a 0-1 modulation providing either a predetermined amplitudeor no amplitude, or, in other words a zero amplitude.

In particular, the amplitude modulated measuring signal may be derivedfrom a measured response signal according to a signal generating phasein which the fluid conduit is provided with a fluid that has apredetermined velocity or is essentially at rest with respect to thefluid conduit.

An impulse signal is applied to the first ultrasonic transducer, such aspiezoelectric transducer, and a response signal of the impulse signal isreceived at a second ultrasonic transducer, such as piezoelectrictransducer.

The measuring signal is derived from the response signal. The derivationof the measuring signal comprises selecting a signal portion of theresponse signal or of a signal derived therefrom and reversing thesignal portion with respect to time and a digitized measuring signal canbe stored in a computer readable memory for later use.

In one particular embodiment, an amplitude of an envelope of themeasuring signal or of a response signal can increase by at least oneorder of magnitude from a leading signal portion of the measuring signalto a maximum amplitude. The leading signal portion precedes the signalmaximum in time. In other words, it is sent out earlier. According tofurther embodiments, the amplitude increases by at least two or even atleast three orders of magnitude.

According to a further aspect, a device for measuring a flow speed in atravel time ultrasonic flow meter is disclosed. The device comprises afirst connector for connecting a first piezoelectric element, a secondconnector for connecting a second piezoelectric element, an optionaldigital to analog converter (DAC), which is connected to the firstconnector and an optional analog to digital converter (ADC), which isconnected to the second connector.

Furthermore, the device comprises a computer readable memory, anelectronic timer or oscillator, a transmitting unit for sending animpulse signal to the first connector and a receiving unit for receivinga response signal to the impulse signal from the second connector.

Moreover, the device comprises means for generating the measuring signalfrom a received response signal, such as a selection unit for selectinga portion of the received response signal or a signal derived therefrom,and an inverting unit for inverting the selected portion of the responsesignal with respect to time to obtain an inverted signal. Optionally, abandpass filter may be provided to remove unwanted signal components.Furthermore, a processing unit is provided for deriving a measuringsignal from at least the inverted signal and for storing the measuringsignal in the computer readable memory.

Furthermore, the device comprises means for measuring a flow speed. Ameasuring signal generator, which is connectable to the first connectoror to the second connector and a transmitting means, such as the DAC andthe connectors, for sending the measuring signal to the first connectorare provided at a sending side. A receiving unit for receiving aresponse signal of the measuring signal from the second connector and avelocity processing unit for deriving a velocity of flow from thereceived response signal are provided at a receiving side. The termsvelocity of flow, flow velocity and flow speed are used as synonyms inthe present application.

While the device can be provided as an analog device without A/D and D/Aconverters and without a computer readable memory unit, it is alsopossible to provide the device or parts of it with a digital computersystem.

In particular, the various signal processing units, such as the velocityprocessing unit, the selection unit and the inverting unit may beprovided entirely or partially by an application specific electroniccomponent or by a program memory with a computer readable instructionset. Similarly, the measuring signal generator and an impulse signalgenerator of the transmitting unit may be provided entirely or partiallyby an application specific electronic component which may comprise acomputer readable instruction set.

According to a further embodiment, the device comprises a direct digitalsignal synthesizer (DDS) that comprises the abovementioned ADC. The DDScomprises a frequency control register, a reference oscillator, anumerically controlled oscillator and a reconstruction low pass filter.Furthermore, the ADC is connectable to the first and to the secondconnector over the reconstruction low pass filter.

Among others, the digital signal synthesizer can be configured tosynthesize a signal, such as the measuring signal, by using apre-determined algorithm or predetermined values which are stored in amemory unit with a computer readable memory. For example, the signal canbe generated by direct signal generation or by DDS (direct digitalsynthesis).

Furthermore, the current specification discloses a flow measurementdevice with a first piezoelectric transducer that is connected to thefirst connector, and with a second ultrasonic transducer, such aspiezoelectric transducer, that is connected to the second connector. Inparticular, the ultrasonic transducers, such as piezoelectrictransducers may be provided with attachment regions, such as a clampingmechanism for attaching them to a pipe.

Furthermore, the current specification discloses a flow measurementdevice with a pipe portion. The first ultrasonic transducer, such aspiezoelectric transducer is mounted to the pipe portion at a firstlocation and the second ultrasonic transducer, such as piezoelectrictransducer is mounted to the pipe portion a second location. Inparticular, the transducers may be clamped to the pipe portion.Providing the device with a pipe portion may provide benefits when thedevice is pre-calibrated with respect to the pipe portion.

The device can be made compact and portable. A portable device accordingto the present specification, which is equipped with surface mountabletransducers, such as clamp-on transducers, can be used to check a pipeon any accessible location. In general, the device may be stationary orportable. Preferentially, the device is sufficiently compact to beplaced at a required location and sufficiently protected againstenvironmental conditions, such as humidity, heat and corrosivesubstances.

Moreover, the current specification discloses a computer readable codefor executing a flow measurement method according to the presentspecification, a computer readable memory comprising the computerreadable code and an application specific electronic component, which isoperable to execute the method steps of a method according to thecurrent specification.

In particular, the application specific electronic component may beprovided by an electronic component comprising the abovementionedcomputer readable memory, such as an EPROM, an EEPROM a flash memory orthe like. According to other embodiments, the application specificelectronic component is provided by a component with a hard-wired orwith a configurable circuitry such as an application specific integratedcircuit (ASIC) or a field programmable gate array (FPGA).

In a further embodiment, an application specific electronic componentaccording to the current specification is provided by a plurality ofinterconnected electronic components, for example by an FPGA, which isconnected to a suitably programmed EPROM in a multi-die arrangement.Further examples of an application specific electronic component areprogrammable integrated circuits such as programmable logic arrays(PLAs) and complex programmable logic devices (CPLDs).

It is helpful to determine whether an off-the-shelf test device ismeasuring a flow speed of a fluid in a fluid conduit according topresent application. To this purpose one provides the fluid conduit witha fluid that has a pre-determined velocity with respect to the fluidconduit. A test impulse signal is applied to a first ultrasonictransducer, such as piezoelectric transducer of the test device, thefirst piezoelectric transducer being mounted to the fluid conduit at afirst location, followed by receiving a test response signal of the testimpulse signal at a second piezoelectric transducer of the test device,the second ultrasonic transducer, such as piezoelectric transducer beingmounted to the fluid conduit at a second location.

A test measuring signal is then derived from the response signal, thederivation of the test measuring signal comprising reversing the signalwith respect to time, followed by comparing the test measuring signalwith a measuring signal that is emitted at the other one of the firstand the second ultrasonic transducer, such as piezoelectric transducer.The measuring signal is a signal that is provided by the test devicewhen supplied by the manufacturer, based on a one-time generated factorymeasuring signal after manufacturing the test device, often mounted to apiece of tube.

In a case where the test device is using a method to determine a flowspeed of a fluid in a fluid conduit according to the application, thetest measuring signal and the measuring signal are similar. In otherwords, reverse engineering of the subject matter of the application isprovided by choosing a test signal and repeating the signal generatingphase of the application until the test measuring signal and themeasuring signal are similar. The term “similar” means that there issignificant correlation between the test measuring signal and themeasuring signal.

The method may also comprise selecting a signal portion of the testresponse signal or of a signal derived therefrom and storing the testmeasuring signal for later use.

Accordingly, a device for measuring a flow speed in a travel timeultrasonic flow meter as defined by functional features comprises afirst connector for a first piezoelectric element, a second connectorfor a second piezoelectric element, a transmitting unit for sending animpulse signal to the first connector, a receiving unit for receiving aresponse signal to the impulse signal from the second connector, aninverting unit for inverting the response signal with respect to time toobtain an inverted signal, a processing unit for deriving a measuringsignal from the inverted signal. When using the device for determining aflow speed of a fluid in a fluid conduit, one will provide the fluidconduit with a fluid that has a velocity with respect to the fluidconduit. This is followed by applying a measuring signal to one of thefirst and the second ultrasonic transducer, such as piezoelectrictransducer, and by measuring a first response signal of the measuringsignal at the other one of the first and the second ultrasonictransducer, such as piezoelectric transducer. One can then derive a flowspeed of the fluid from the first response signal. Reverse engineeringof the device will reveal that, when applying a test impulse signal to afirst ultrasonic transducer, such as piezoelectric transducer of thetest device, receiving a test response signal of the test impulse signalat a second piezoelectric transducer of the test device, the secondultrasonic transducer, such as piezoelectric transducer being mounted tothe fluid conduit at a second location, deriving a test measuring signalfrom the response signal, the derivation of the test measuring signalcomprising reversing the signal with respect to time, wherein the testmeasuring signal and a measuring signal that is emitted at the first orthe second ultrasonic transducer, such as piezoelectric transducer aresimilar. This functional description helps to characterize the device ofthe application without describing the structure and shape of theemitted signals.

It is clear that the device can have a D/A converter, the D/A converterbeing connected to the first connector, an A/D converter, the A/Dconverter being connected to the second connector, and a computerreadable memory. It can further comprise a selection unit for selectinga portion of the received response signal or a signal derived therefrom,wherein the evaluations above are done with the selected portion of thereceived response signal or a signal derived therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present specification is now explained infurther detail with respect to the following Figures, wherein:

FIG. 1 shows a first flow meter arrangement with two piezoelectricelements.

FIG. 2 shows the flow meter arrangement of FIG. 1 with one directsignal.

FIG. 3 shows the flow meter arrangement of FIG. 2 in the viewingdirection A-A.

FIG. 4 shows a second flow meter arrangement with four piezoelectricelements and four direct signals.

FIG. 5 shows the flow meter arrangement of FIG. 4 in the viewingdirection B-B.

FIG. 6 shows a schematic diagram of a test signal.

FIG. 7 shows a schematic diagram of a test signal response.

FIG. 8 shows a schematic diagram of an inverted signal.

FIG. 9 shows a schematic diagram of a response from the inverted signal.

FIG. 10 shows a first inverted signal in high resolution.

FIG. 11 shows a response of the inverted signal of FIG. 10.

FIG. 12 shows a further inverted signal in high resolution.

FIG. 13 shows a response of the inverted signal of FIG. 12.

FIG. 14 shows a further inverted signal in high resolution.

FIG. 15 shows a response of the inverted signal of FIG. 14.

FIG. 16 shows a further inverted signal in high resolution.

FIG. 17 shows a response of the inverted signal of FIG. 16.

FIG. 18 shows a further inverted signal in high resolution.

FIG. 19 shows a response of the inverted signal of FIG. 18.

FIG. 20 shows a further inverted signal in high resolution.

FIG. 21 shows a response of the inverted signal of FIG. 20.

FIG. 22 shows a further inverted signal in high resolution.

FIG. 23 shows a response of the inverted signal of FIG. 22.

FIG. 24 shows a further inverted signal in high resolution.

FIG. 25 shows a response of the inverted signal of FIG. 24.

FIG. 26 shows a further inverted signal in high resolution.

FIG. 27 shows a response of the inverted signal of FIG. 26.

FIG. 28 shows a further inverted signal in 12-bit resolution.

FIG. 29 shows a response of the signal of FIG. 28.

FIG. 30 shows a further inverted signal in 3-bit resolution.

FIG. 31 shows a response of the signal of FIG. 30.

FIG. 32 shows a further inverted signal in 2-bit resolution.

FIG. 33 shows a response of the signal of FIG. 32.

FIG. 34 shows a further inverted signal in 1-bit resolution.

FIG. 35 shows a response of the signal of FIG. 34.

FIG. 36 shows a short impulse at a piezoelectric element of the flowmeter of FIG. 1.

FIG. 37 shows a signal of a piezoelectric element of the flow meter ofFIG. 1, which is derived from the inverted response of the signal ofFIG. 36.

FIG. 38 shows a response of the signal of FIG. 37.

FIG. 39 shows an upstream and a downstream cross correlation function.

FIG. 40 shows a sectional enlargement of FIG. 39.

FIG. 41 shows a schematic diagram of a device for measuring a flow speedaccording to the present specification.

FIG. 42 shows a schematic diagram of a direct digital synthesizer foruse in the device of FIG. 41.

FIG. 43 shows a first multi-transducer arrangement. and

FIG. 44 shows a second multi-transducer arrangement.

FIG. 45 shows a Z-configuration of clamp-on transducers.

FIG. 46 shows a V-configuration of clamp-on transducers.

FIG. 47 shows a W-configuration of clamp-on transducers.

FIG. 48 shows a one-cycle sending signal.

FIG. 49 shows a ten cycle sending signal.

FIG. 50 shows a TRA sending signal.

FIG. 51 shows a response signal to the one-cycle sending signal of FIG.48.

FIG. 52 shows a response signal to the ten cycle sending signal of FIG.49.

FIG. 53 shows a response signal to the TRA sending signal of FIG. 50.

FIG. 54 shows a pressure curve of a TRA sending signal and a responsesignal to the TRA sending signal.

FIG. 55 shows a pressure curve of a TRA sending signal and a responsesignal to the TRA sending signal.

FIG. 56 shows an impulse signal that is used to generate the signalinput of FIG. 55.

FIG. 57 shows a first response signal indicating channel properties.

FIG. 58 shows a second response signal indicating channel properties.

FIG. 59 shows a further response signal.

FIG. 60 shows a further response signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, details are provided to describe theembodiments of the present specification. It shall be apparent to oneskilled in the art, however, that the embodiments may be practisedwithout such details.

FIG. 1 shows a first flow meter arrangement 10. In the flow meterarrangement, a first piezoelectric element 11 is placed at an outer wallof a pipe 12, which is also referred as a tube 12. A secondpiezoelectric element 13 is placed at an opposite side of the pipe 12such that a direct line between the second piezoeloelectric element 11and the downstream piezoelectric element 13 is oriented at an angle β tothe direction 14 of average flow, which is at the same time also thedirection of the pipe's 12 symmetry axis. The angle β is chosen to beapproximately 45 degrees in the example of FIG. 1 but it may also besteeper, such as for example 60 degrees, or shallower, such as forexample 30 degrees.

A piezoelectric element, such as the piezoelectric elements 11, 13 ofFIG. 1 may in general be operated as an acoustic transmitter and as anacoustic sensor. An acoustic transmitter and an acoustic sensor may beprovided by the same piezoelectric element or by different regions ofthe same piezoelectric element. In this case, a piezoelectric element ortransducer is also referred to as piezoelectric transmitter when it isoperated as transmitter or sound source and it is also referred to asacoustic sensor or receiver when it is operated as acoustic sensor.

When a flow direction is as shown in FIG. 1, the first piezoelectricelement 11 is also referred to as “upstream” piezoelectric element andthe second piezoelectric element 13 is also referred to as “downstream”piezoelectric element. A flow meter according to the presentspecification works for both directions of flow in essentially the sameway and the flow direction of FIG. 1 is only provided by way of example.

FIG. 1 shows a flow of electric signals of FIG. 1 for a configuration inwhich the upstream piezoelectric element 11 is operated as apiezoelectric transducer and the downstream piezoelectric element 13 isoperated as an acoustic sensor. For the purpose of clarity, theapplication works upstream and downstream, i.e. the position of thepiezoelectric elements can be interchanged.

A first computation unit 15 is connected to the upstream piezoelectricelement 11 and a second computation unit 16 is connected to thedownstream piezoelectric element 13. The first computation unit 15comprises a first digital signal processor, a first digital analogconverter (DAC) and a first analog digital converter (ADC). Likewise,the second computation unit 16 comprises a second digital signalprocessor, a second digital analog converter (DAC) and a second analogdigital converter (ADC). The first computation unit 15 is connected tothe second computation unit 16.

The arrangement with two computation units 15, 16 shown in FIG. 1 isonly provided by way of example. Other embodiments may have differentnumbers and arrangements of computation units. For example, there may beonly one central computation unit or there may be two AD/DC convertersand one central computation unit, or there may be two small-scalecomputation units at the transducers and one larger central computationunit.

A computation unit or computation units can be provided bymicrocontrollers or application specific integrated circuits (ASICs),ACIDs or field programmable gate arrays (FPGAs), for example.Specifically, the synthesis of an electrical signal from a storeddigital signal may be provided by a direct digital synthesizer (DDS),which comprises a digital to analog converter (DA, DAC).

A method for generating a measuring signal according to the presentspecification comprises the following steps.

A pre-determined digital test signal is generated by synthesizing anacoustic signal with the digital signal processor of the firstcomputation unit 15. The digital test signal is sent from the firstcomputation unit 15 to the piezoelectric transducer 11 along signal path17. The piezoelectric transducer 11 generates a corresponding ultrasoundtest signal. Units 15 and 16 can also be provided in one single unit.

The test signal is provided as a short pulse, for example by a single 1MHz oscillation or by 10 such oscillations. In particular, the testsignal may be provided by a small number of oscillations with constantamplitude, thereby approximating a rectangular signal. The oscillationor the oscillations may have a sinusoidal shape, a triangular shape, arectangular shape or also other shapes.

The ultrasound test signal travels through the liquid in the pipe 12 tothe piezoelectric sensor 13. In FIG. 1, a direct signal path of theultrasound signal is indicated by an arrow 18. Likewise, a direct signalpath of the ultrasound signal in the reverse direction is indicated byan arrow 19. A response signal is picked up by the piezoelectric sensor13, sent to the second computation unit 16 along signal path 20, anddigitized by the second computation unit 16.

In a further step, a digital measuring signal is derived from thedigitized response signal. The derivation of the measurement a reversalof the digitized response signal with respect to time. According tofurther embodiments, the derivation comprises further steps such as aconversion to a reduced resolution in the amplitude range, a bandwidthfiltering of the signal to remove noise, such as low frequency noise andhigh frequency noise. In particular, the step of bandwidth filtering maybe executed before the step of reversing the signal with respect totime.

The signal reversal may be carried out in various ways, for example byreading out a memory area in reverse direction or by reversing the signof sinus components in a Fourier representation.

In one embodiment, a suitable portion of the digitized response signalis selected that contains the response from the direct signal. Theportion of the response signal is then turned around, or inverted, withrespect to time. In other words, signal portions of the response signalthat are received later are sent out earlier in the inverted measuringsignal. If a signal is represented by a time ordered sequence ofamplitude samples, by way of example, the abovementioned signalinversion amounts to inverting or reversing the order of the amplitudesamples.

The resulting signal, in which the direction, or the sign, of time hasbeen inverted, is also referred to as an “inverted signal”. Theexpression “inverted” in this context refers to an inversion withrespect to the direction of time, and not to an inversion with respectto a value, such as the amplitude value.

FIGS. 10 to 19 show, by way of example digital signals according to thepresent specification.

In a flow meter according to one embodiment of the presentspecification, the same measuring signal is used for both directions 18,19, the downstream and the upstream direction, providing a simple andefficient arrangement. According to other embodiments, differentmeasuring signals are used for both directions. In particular, themeasuring signal may be applied to the original receiver of the testsignal. Such arrangements may provide benefits for asymmetric conditionsand pipe shapes.

A method of measuring a flow speed of a liquid through a pipe, whichuses the abovementioned inverted signal as a measuring signal, comprisesthe following steps.

The abovementioned measuring signal is sent from the first computationunit 15 to the piezoelectric transducer 11 along signal path 17. Thepiezoelectric transducer 11 generates a corresponding ultrasoundmeasuring signal. Examples for such a measuring signal are provided inFIGS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 37 and 38.

The ultrasound measuring signal travels through the liquid in the pipe12 to the piezoelectric sensor 13. A response signal is picked up by thepiezoelectric sensor 13, sent to the second computation unit 16 alongsignal path 20, and digitized by the second computation unit 16.

The second computation unit 16 sends the digitized response signal tothe first computation unit 15. The first computation unit 15 determinesa time of flight of the received signal, for example by using one of themethods described further below.

A similar process is carried out for a signal travelling in the reversedirection 19, namely the abovementioned measuring signal is applied tothe downstream piezoelectric element 13 and a response signal ismeasured by the upstream piezoelectric element 11 to obtain an upstreamtime of flight TOF_up in the reverse direction 19. The first computationunit 15 determines a velocity of flow, for example according to theformula

${v = {\frac{c^{2}}{{2 \cdot L \cdot \cos}\;\beta} \cdot \left( {{TOF}_{up} - {TOF}_{down}} \right)}},$

wherein L is the length of the direct path between the piezoelectricelements 11, 13, β is the angle of inclination of the direct pathbetween the piezoelectric elements 11, 13 and the direction of theaverage flow, and c is the velocity of sound in the liquid under thegiven pressure and temperature conditions.

The squared velocity of sound c{circumflex over ( )}2 can beapproximated to second order by the expression

$c^{2} \approx \frac{L^{2}}{{TOF}_{up}*{TOF}_{down}}$

-   -   which leads to the formula

$v = {\frac{L}{2*\cos\;\beta} \cdot \frac{{TOF}_{up} - {TOF}_{down}}{{TOF}_{up}*{TOF}_{down}}}$

Thereby, it is not necessary to determine temperature or pressure, whichin turn determine the fluid density and the sound velocity, or tomeasure the sound velocity or the fluid density directly. By contrast,the first order of the error does not cancel out for only onemeasurement direction.

Instead of using a factor 2·L·cos β, a proportionality constant can bederived from a calibration measurement with a known flow speed. Theproportionality constant of the calibration takes into account furthereffects such as flow profiles and contributions from sound waves thatwere scattered and did not travel along a straight line.

According to a further embodiment, the process of generating an impulsesignal, recording a response signal and deriving an inverted measuringsignal from the response signal is simulated in a computer. Relevantparameters, such as the pipe diameter of the pipe 12 and the sensorplacements are provided as input parameters to the simulation.

According to yet another embodiment, the measuring signal, which is tobe supplied to a transmitting piezoelectric element, is synthesizedusing a shape of a typical response signal to an impulse signal, such asthe signal shapes shown in FIGS. 37 and 38. For example, the measuringsignal may be provided by a 1 MHz sinusoidal oscillation, which isamplitude modulated with an envelope according to a Gaussian probabilityfunction having a half width of 10 microseconds. The half-width may bechosen as an input parameter, which depends on the actual arrangement,such as the pipe diameter and the sensor placement.

A flow meter according to the present specification may also be providedas a pre-defined flow meter in which the measuring signal is generatedduring a test run at a factory site, in particular when the flow meteris supplied together with a pipe section.

According to a simple embodiment of the present specification, a time offlight in upstream and in downstream direction is determined byevaluating a time of a peak amplitude of a received signal with respectto a sending time of the measuring signal. To achieve a higherprecision, the maximum may be determined using an envelope of thereceived signal. According to a further embodiment, the measurement isrepeated multiple times and an average time of flight is used.

According to a further embodiment of the present specification, the timeof flight of a signal is evaluated using a cross-correlation technique.In particular, the respective time shifts can be evaluated bycross-correlating the received downstream or upstream signal with thereceived signal at zero flow speed according to the formula:

${{{CCorr}(\tau)} = {\sum\limits_{t = {- \infty}}^{\infty}{{{Sig}_{Flow}(t)} \cdot {{Sig}_{NoFlow}\left( {t + \tau} \right)}}}},$

wherein Sig_Flow represents an upstream or downstream signal undermeasurement conditions, when there is a fluid flow through the pipe, andwherein Sig_NoFlow represents a signal under calibration conditions atzero flow. The infinite sum limits represent a sufficiently large timewindow [−T1, +T2]. In more general words, −T1 and +T2 do not need to besame and for practical reasons this can be advantageous for the flowmeter.

The time shift TOF_up−TOF_down is then obtained by comparing the time ofthe maximum of the upstream correlation function with the time of themaximum of the downstream correlation function. The envelope of thecorrelation function may be used to determine the location of themaximum more accurately.

In a further embodiment, a separate evaluation unit is provided betweenthe first computation unit 15 and the second computation unit 16, whichperforms the calculation of the signal arrival times and the flow speed.

In general, the measured signal of the acoustic sensor results from asuperposition of scattered signals and a direct signal. The scatteredsignals are scattered from the walls of the pipe once or multiple times.This is shown, by way of example, in FIGS. 2 and 3.

The transducer configuration of FIG. 1 is a direct-line or “Z”configuration. Other arrangements, which make use of reflections on anopposite side of the pipe, are possible as well, such as the “V” and the“W” configuration. V and W configuration work based on reflections onthe pipe wall which induce more scatterings than the Z configuration.The subject matter of the application will benefit from theseconfigurations as long as the paths are understood properly.

In a V-configuration, the two transducers are mounted on the same sideof the pipe. For recording a 45 degree reflection they are placed abouta pipe diameter apart in the direction of the flow. The W-configurationmakes use of three reflections. Similar to the V-configuration, the twotransducers are mounted on the same side of the pipe. For recording asignal after two 45 degree reflections they are placed two pipediameters apart in the direction of the flow.

FIG. 2 shows, by way of example a first acoustic signal which travelsdirectly from the piezoelectric element 11 to the piezoelectric element13,

For simplicity, the scattering events are shown as reflections in FIGS.2 to 5 but the actual scattering process can be more complicated. Inparticular, the most relevant scattering occurs typically on the pipewall or at material that is mounted in front of the piezoelectrictransducers. The received scattering also depends on the sensorarrangement. By way of example, FIGS. 45, 46, and 47 show Z, V, and Wsensor arrangements. FIG. 3 shows a view of FIG. 2 in flow direction inthe viewing direction A-A.

FIGS. 4 and 5 show a second sensor arrangement in which a furtherpiezoelectric element 22 is positioned at a 45 degree angle to thepiezoelectric element 11 and a further piezoelectric element 23 ispositioned at a 45 degree angle to the piezoelectric element 13.

Furthermore, FIGS. 4 and 5 show direct, or straight line, acousticsignal paths for a situation in which the piezoelectric elements 11, 22are operated as piezo transducers and the piezoelectric elements 13, 23are operated as acoustic sensors. Piezoelectric element 23, which is onthe back of the pipe 12 in the view of FIG. 4 is shown by a dashed linein FIG. 4.

FIGS. 6 to 9 show, in a simplified way, a method of generating ameasuring signal from a response of a test signal. In FIGS. 6 to 9,losses due to scattering are indicated by hatched portions of a signaland by arrows.

For the considerations of FIGS. 6 to 9 it is assumed that the acousticsignal only propagates along a straight line path, along a firstscattering channel with a time delay of Δt, and along a secondscattering channel with a time delay of 2Δt. Signal attenuation alongthe paths is not considered.

A test signal in the form of a rectangular spike is applied to thepiezoelectric element 11. Due to scattering, a first portion of thesignal amplitude is lost due to the first scattering path and appearsafter a time Δt, and a second portion of the signal amplitude is lostdue to the second scattering path and appears after a time 2Δt. Thisyields a signal according to the white columns in FIG. 7, which isrecorded at the piezoelectric element 13.

A signal processor inverts this recorded signal with respect to time andit applies the inverted signal to the piezoelectric element 11. The samescattering process as explained before now applies to all three signalcomponents. As a result, a signal according to FIG. 9 is recorded at thepiezoelectric element 13, which is approximately symmetric.

In reality, the received signals will be distributed over time and thereoften is a “surface wave” which has travelled through material of thepipe and arrives before the direct signal. This surface wave isdiscarded by choosing a suitable time window for generating the invertedmeasuring signal. Likewise, signals that stem from multiple reflectionsand arrive late can be discarded by limiting the time window and/or bychoosing specific parts of the signal.

The following table shows measured time delays for a direct alignment,or, in other words, for a straight line connection between clamped-onpiezoelectric elements on a DN 250 pipe in a plane perpendicular to thelongitudinal extension of the DN 250 pipe. The flow rate refers to aflow of water through the DN 250 pipe.

Herein “TOF 1 cycle” refers to an impulse such as the one shown in FIG.36, that is generated by a piezoelectric element, which is excited by anelectric signal with 1 oscillation having a 1 μs period. “TOF 10 cycle”refers to a signal that is generated by a piezoelectric element, whichis excited by an electric signal with 10 sinusoidal oscillations ofconstant amplitude having a 1 μs period.

Flowrate/Method 21 m³/h 44 m³/h 61 m³/h TOF 1 cycle 7 ns 18 ns 27 ns TOF10 cycle 9 ns 19 ns 26 ns Time reversal 8 ns 18 ns 27 ns

FIGS. 10-27 show high resolution inverted signals and their respectiveresponse signals. The voltage is plotted in arbitrary units over thetime in microseconds.

The time axes in the upper Figures show a transmitting time of theinverted signal. The transmitting time is limited to the time windowthat is used to record the inverted signal. In the example of FIGS.10-27 the time window starts shortly before the onset of the maximum,which comes from the direct signal and ends 100 microseconds thereafter.

The time axes in the lower Figures are centered around the maximum ofthe response signals and extend 100 microseconds, which is the size ofthe time window for the inverted signal, before and after the maximum ofthe response signals.

FIGS. 28-35 show digitized inverted signals in a high resolution and in12, 3, 2 and 1 bit resolution in the amplitude range and theirrespective response signals. The voltage is plotted in Volt over thetime in microseconds. The signals of FIG. 28-35 were obtained for awater filled DN 250 pipe.

The length of the time window for the inverted signal is 450microseconds. Hence, the time window of FIGS. 28-35 is more than fourtimes larger than in the preceding FIGS. 90-27.

In FIGS. 28-35 it can be seen that even a digitization with 1 bitresolution produces a sharp spike. It can be seen that the spike becomeseven more pronounced for the lower resolutions. A possible explanationfor this effect is that in the example of FIGS. 28-35 the total energyof the input signal is increased by using a coarser digitization in theamplitude range while the response signal remains concentrated in time.

FIG. 36 shows a signal that is generated by a piezoelectric elementafter receiving an electric pulse that lasts for about 0.56microseconds, which is equivalent to a frequency of 3.57 MHz. Due to theinertia of the piezoelectric element, the maximum amplitude for thenegative voltage is smaller than for the positive voltage and there aremultiple reverberations before the piezoelectric element comes to rest.

FIG. 37 shows an electric signal that is applied to a piezoelectricelement, such as the upstream piezoelectric element 11 of FIG. 1. Thesignal of FIG. 37 is derived by forming an average of ten digitizedresponse signals to a signal of the type shown in FIG. 36 and timereversing the signal, wherein the response signals are received by apiezoelectric element such as the downstream piezoelectric element 13 ofFIG. 1.

In the example of FIG. 37, the digitized signals are obtained by cuttingout a signal portion from the response signal that begins approximately10 microseconds before the onset of envelope of the response signal andthat ends approximately 55 microseconds behind the envelope of theresponse signal. The envelope shape of the response signal of FIG. 37 issimilar to the shape of a Gaussian probability distribution, or, inother words, to a suitable shifted and scaled version ofexp(−x{circumflex over ( )}2).

FIG. 38 shows a portion of a response signal to the signal shown in FIG.37, wherein the signal of FIG. 37 is applied to a first piezoelectricelement, such as the upstream piezoelectric element 11, and received ata second piezoelectric element, such as the downstream piezoelectricelement 13 of FIG. 1.

FIG. 39 shows a an upstream cross correlation function and a downstreamcross correlation function, which are obtained by cross correlating theupstream signal and the downstream signal of the arrangement of FIG. 1with a signal obtained at zero flow, respectively.

FIG. 40 shows a sectional enlargement of FIG. 39. Two position markersindicate the positions of the respective maxima of the upstream anddownstream cross correlation function. The time difference between themaxima is a measure for the time difference between the upstream and thedownstream signal.

FIGS. 48, 49 and 50 show three different sending signals: FIG. 48 showsa conventional pulse (1 cycle) and FIG. 48 shows a 10 cycles pulsecompared to the measuring signal generated by as described above, suchas the signal of FIG. 50. The transducers have been clamped onto a DN250pipe.

FIGS. 51, 52 and 53 show the corresponding received signals aftersending the signals of illustrated in the respective FIGS. 48, 59 and50. By comparison it can be easily seen that measuring signal focus theenergy and generates a more than two times larger amplitude of thereceiving signal compared to the receiving signals in response to theconventional pulses (e.g. 1 or 10 cycles) of FIGS. 48 and 49.

FIG. 41 shows, by way of example, a flow measurement device 60 formeasuring a flow in the arrangement in FIG. 1 or other arrangementsaccording to the specification. In the arrangement of FIG. 1, the flowmeasurement device 60 is provided by the first and second computationunits 15, 16.

The flow measurement device 60 comprises a first connector 61 forconnecting a first piezoelectric transducer and a second connector 62for connecting a second piezoelectric transducer. The first connector 61is connected to a digital to analog converter (DAC) 64 over amultiplexer 63. The second connector 62 is connected to an analog todigital converter 65 over a demultiplexer 66.

The ADC 65 is connected to a signal selection unit 67, which isconnected to a signal inversion unit 68, which is connected to a bandpass filter 69, which is connected to a computer readable memory 70.Furthermore, the ADC 65 is connected to a velocity computation unit 71.

The DAC 64 is connected to an impulse signal generator 72 and ameasuring signal generator 73. The measuring signal generator isconnected to the impulse generator 72 over a command line 74. Thevelocity computation unit 71 is connected to the measuring signalgenerator 73 via a second command line 75.

In general, the impulse signal generator 72 and the measuring signalgenerator comprise hardware elements, such as an oscillator, andsoftware elements, such as an impulse generator module and a measuringsignal generator module. In this case, the command lines 74, 75 may beprovided by software interfaces between respective modules.

During a signal generating phase, the impulse signal generator sends asignal to the DAC 64, the selection unit 67 receives a correspondingincoming signal over the ADC 65 and selects a portion of an incomingsignal. The inversion unit 68 inverts the selected signal portion withrespect to time, the optional bandpass filter 69 filters out lower andupper frequencies and the resulting measuring signal is stored in thecomputer memory 70. When the word “signal” is used with reference to asignal manipulation step, it may in particular refer to a representationof a signal in a computer memory.

In particular, a signal representation can be defined by value pairs ofdigitized amplitudes and associated discrete times. Otherrepresentations comprise, among others, Fourier coefficients, waveletcoefficients and an envelope for amplitude modulating a signal.

FIG. 42 shows a second embodiment of a flow measurement device 60′ formeasuring a flow in the arrangement in FIG. 1 or other arrangementsaccording to the specification. The flow measurement device 60′comprises a direct digital synthesizer (DDS) 76. For simplicity, onlythe components of the DDS 76 are shown. The DDS 76 is also referred toas an arbitrary waveform generator (AWG).

The DDS 76 comprises a reference oscillator 77, which is connected to afrequency controller register 78, a numerically controlled oscillator(NCO) 79 and to the DAC 64. An input of the NCO 79 for N channels isconnected to an output of the frequency control register 78. An input ofthe DAC 64 for M channels is connected to the NCO 79 and an input of areconstruction low pass filter is connected to the DAC 64. By way ofexample, a direct numerically controlled oscillator 79 with a clockfrequency of 100 MHz may be used to generate an amplitude modulated 1MHz signal.

An output of the reconstruction low pass filter 80 is connected to thepiezoelectric transducers 11, 13 of FIG. 1.

Due to the inertia of an oscillator crystal, it is often advantageous touse an oscillator with a higher frequency than that of a carrier wave inorder to obtain a predetermined amplitude modulated signal, for exampleby using a direct digital synthesizer.

FIGS. 45, 47 and 48 illustrate the abovementioned Z, V and W flowmeasurement configurations. In the examples of FIGS. 45, 47, 48 clamp-ontransducers are attached to a conduit via respective coupling pieces.

FIGS. 54 and 55 show a comparison of respective receiving or responsesignals to respective sending signals that were generated without usinga time reversal procedure and with the use of a time reversal procedure.

In the example of FIG. 54, a modulated sine wave with a Gaussian shapedenvelope is used as a sending signal. The signal energy of the sendingsignal is proportional to 1.3×10⁻⁷ (Pa/m)² s and the signal amplitude is0.1 Pa. The value is obtained by integrating the squared pressure perunit length over time. The response signal has a peak-to-peak amplitudeof the receiving signal of about 0.09 Pa.

In the example of FIG. 55, a time reversed signal, which is derived fromthe response signal to the impulse signal of FIG. 56, is used as asending signal. The sending signal is adjusted to have the same signalenergy of 1.3×10⁻⁷ (Pa/m)² s as the sending signal of FIG. 54. Thisyields a peak-to-peak amplitude of the receiving signal which is about0.375 Pa.

The receiving amplitude of FIG. 55 is more than four times higher thanthe amplitude of the receiving signal of FIG. 54. The increasedamplitude on the receiving side can provide easier and more stablesignal recognition. Among others, the increase in amplitude can beadjusted by adjusting the bit resolution of the amplitude of the timereversed signal, in particular by increasing or decreasing thebit-resolution in order to obtain a larger amplitude.

FIGS. 56 and 57 illustrate how the receiving signals can be used toderive information about the transmission channel and in particularabout the wall thickness of the conduit, deposits on the wall. Accordingto the present specification, a response to the measuring signal, whichis the time reversed response signal, can be analysed to allow adetermination of property changes of the pipe material, like cracks,crustification, etc. In a flow measurement according to one embodimentof the present specification, these property changes are determined byanalysing the same receiving signal that is used for the time of flightmeasurement.

FIG. 57 shows a first response signal, which contains information abouta first transmission channel.

FIG. 58 shows a second response signal, which contains information abouta second transmission channel. The length of the horizontal arrow on thecentral main lobe extends between the left side lobe and the right sidelobe, which are left and right to the main lobe, respectively. Thelength of the arrow represents the thickness of a pipe wall if thesignal is generated according to the FIG. 46. The measured wallthickness is determined at the location where the wave is reflected atthe lower part of the pipe in FIG. 46. If there is a deposit on the pipewall, the measured wall thickness will increase.

FIG. 59 shows a further response signal. The experimental setup forobtaining the signal of FIG. 59 comprises clamp-on, angle transducers,an acrylic transducer coupling head, a sound velocity of c=2370 m/s, acoupling angle of 40°, a stainless steel wall, a transversal wavevelocity of c=3230 m/s, 61.17°, water as fluid, a sound velocity in thefluid of c=1480 m/s, a transversal wave angle axis of 23.67°, and a flowangle of 66.33°, extracted from FIG. 59

FIG. 60 shows a further response signal. The experimental setup forobtaining the signal of FIG. 60 comprises an acrylic transducer couplinghead, a sound velocity of c=2370 m/s, a coupling angle of 20°, astainless steel wall, longitudinal wave velocity of c=5790 m/s, 56.68°,transversal wave c=3230 m/s, water as fluid, a sound velocity in thefluid of c=1480 m/s, a longitudinal wave angle axis=12.33°, atransversal wave angle axis of 12.33°, and a flow angle of 77.67°,extracted from FIG. 60.

The alternative set-up configurations for FIGS. 59 and 60 are shown inFIGS. 45, 46 and 47.

According to one embodiment of the present specification the channelproperties are deduced by analysing a receiving signal such as thesignals of FIGS. 57 to 60.

The example of FIGS. 59 and 60 illustrates the differences in thereceiving signals depending on the presence of longitudinal andtransversal waves in the pipe material. The presence of these waves aretypical for the selected material and the geometry and can be used formaterial analysis. Such material analysis based on ultrasonic test wavesare used in the application field of Non-Destructive Testing (NDT). Thispresent specification allows the simultaneous analysis of flow and e.g.piping material as the received signal contains the impulse response ofthe measurement system including the transmission channel and materialenvironment.

The analysis of the receiving signals can be carried out in variousways, such as comparing the receiving signal with a previously receivedimpulse response or direct evaluation of an impulse response, forexample for determining a wall thickness.

Although the above description contains much specificity, these shouldnot be construed as limiting the scope of the embodiments but merelyproviding illustration of the foreseeable embodiments. The method stepsmay be performed in different order than in the provided embodiments,and the subdivision of the measurement device into processing units andtheir respective interconnections may be different from the providedembodiments.

In particular, the method steps of storing a digital representation of asignal and performing operations such as selection a signal portion,time reversing a signal and filtering a signal may be interchanged. Forexample, a signal may be stored in a time inverted form or it may beread out in reverse order to obtain a time inverted signal.

While the present disclosure is explained with respect to a round DN 250pipe, it can be readily applied to other pipe sizes or even to otherpipe shapes. Although the embodiments are explained with respect toclamp-on transducers, wet transducers, which protrude into a pipe orinstalled in an open channel, may be used as well.

Especially, the above stated advantages of the embodiments should not beconstrued as limiting the scope of the embodiments but merely to explainpossible achievements if the described embodiments are put intopractise. Thus, the scope of the embodiments should be determined by theclaims and their equivalents, rather than by the examples given.

The embodiments of the present specification can also be described withthe following lists of elements being organized into embodiments. Therespective combinations of features which are disclosed in theembodiment list are regarded as independent subject matter,respectively, that can also be combined with other features of theapplication.

Embodiment 1: A method for determining a flow speed of a fluid in afluid conduit comprising:

-   -   providing the fluid conduit with a fluid that has a        predetermined velocity with respect to the fluid conduit,    -   applying an impulse signal to a first ultrasonic transducer, the        first ultrasonic transducer being mounted to the fluid conduit        at a first location,    -   receiving a response signal of the impulse signal at a second        ultrasonic transducer, the second ultrasonic transducer being        located at the fluid conduit at a second location,    -   deriving a measuring signal from the response signal, the        derivation of the measuring signal comprising selecting a signal        portion of the response signal or of a signal derived therefrom        and reversing the signal portion with respect to time,    -   storing the measuring signal for later use,    -   providing the fluid conduit with the fluid, the fluid moving        with respect to the fluid conduit,    -   applying the measuring signal to one of the first and the second        ultrasonic transducers,    -   measuring a first response signal of the measuring signal at the        other one of the first and the second ultrasonic transducer,    -   deriving a flow speed of the fluid from the first response        signal,

wherein the following steps of

-   -   applying an impulse signal to a first ultrasonic transducer, the        first ultrasonic transducer being mounted to the fluid conduit        at a first location,    -   receiving a response signal of the impulse signal at a second        ultrasonic transducer, the second ultrasonic transducer being        located at the fluid conduit at a second location,    -   deriving a measuring signal from the response signal, the        derivation of the measuring signal comprising selecting a signal        portion of the response signal or of a signal derived therefrom        and reversing the signal portion with respect to time,    -   storing the measuring signal for later use, are optional or can        be left away if the measurement signal has been established        earlier.

Embodiment 2: The method according to embodiment 1, comprising

-   -   repeating the steps of applying the measuring signal and        measuring the response signal in the reverse direction to obtain        a second response signal,    -   deriving a flow speed of the fluid from the first response        signal and the second response signal.

Embodiment 3: The method according to embodiment 1 or embodiment 2,wherein the signal portion that is used to derive the measuring signalcomprises a first portion around a maximum amplitude of the responsesignal and a trailing signal portion, the trailing signal portionextending in time behind the arrival time of the maximum amplitude.

Embodiment 4: The method according to one of the preceding embodiments,comprising

-   -   repeating the steps of applying an impulse signal and receiving        a corresponding response signal multiple times, thereby        obtaining a plurality of response signals,    -   deriving the measuring signal from an average of the received        response signals.

Embodiment 5: The method according to one of the preceding embodiments,

-   -   wherein the derivation of measuring signal comprises digitizing        the response signal or a signal derived therefrom with respect        to amplitude.

Embodiment 6: The method according to embodiment 5, comprisingincreasing the bit-resolution of the digitized signal for increasing anamplitude of a response signal to the measuring signal.

Embodiment 7: The method according to embodiment 5, comprisingdecreasing the bit-resolution of the digitized signal for increasing anamplitude of a response signal to the measuring signal.

Embodiment 8: The method according to one of the embodiments 5 to 7,wherein the bit resolution of the digitized signal with respect to theamplitude is a low bit resolution.

Embodiment 9: The method according to one of the preceding embodiments,comprising processing of at least one of the response signals fordetermining a change in the wall thickness of the conduit or fordetermining material characteristics of the conduit walls by determininglongitudinal and transversal sound wave characteristics.

Embodiment 10: A device for measuring a flow speed in a travel timeultrasonic flow meter, comprising

-   -   a first connector for a first ultrasonic element,    -   a second connector for a second ultrasonic element,    -   a transmitting unit for sending an impulse signal to the first        connector,    -   a receiving unit for receiving a response signal to the impulse        signal from the second connector,    -   an inverting unit for inverting the response signal with respect        to time to obtain an inverted signal,    -   a processing unit for deriving a measuring signal from the        inverted signal and storing the measuring signal, wherein the        following elements of        -   a transmitting unit for sending an impulse signal to the            first connector,        -   a receiving unit for receiving a response signal to the            impulse signal from the second connector,        -   an inverting unit for inverting the response signal with            respect to time to obtain an inverted signal,        -   a processing unit for deriving a measuring signal from the            inverted signal and storing the measuring signal, are            optional or can be left away if the measurement signal has            been established earlier so that it is readily available.

Embodiment 11: The device of embodiment 10, further comprising:

-   -   a D/A converter, the D/A converter being connected to the first        connector,    -   an A/D converter, the A/D converter being connected to the        second connector,    -   a computer readable memory for storing the measuring signal.

Embodiment 12: The device of embodiment 10 or embodiment 11, furthercomprising a selection unit for selecting a portion of the receivedresponse signal or a signal derived therefrom, wherein the invertingunit is provided for inverting the selected portion of the responsesignal with respect to time to obtain the inverted signal.

Embodiment 13: The device of one of embodiments 10 to 12, the devicecomprising

-   -   a measuring signal generator, the measuring signal generator        being connectable to the first connector or to the second        connector,    -   a transmitting means for sending the measuring signal to the        first connector,    -   a receiving unit for receiving a response signal of the        measuring signal from the second connector,    -   a second processing unit for deriving a flow speed from the        received response signal.

Embodiment 14: The device according to one of the embodiments 10 to 13,the device comprising:

-   -   a direct digital signal synthesizer, the direct digital signal        synthesizer comprising the ADC,    -   a frequency control register, a reference oscillator, a        numerically controlled oscillator and a reconstruction low pass        filter, the ADC being connectable to the first and the second        connector over the reconstruction low pass filter.

Embodiment 15: The device according to one of the embodiments 10 to 14,the device comprising:

-   -   a first ultrasonic transducer, the first ultrasonic transducer        being connected to the first connector,    -   a second ultrasonic transducer, the second ultrasonic transducer        being connected to the second connector.

Embodiment 16: The device according to one of the embodiments 10 to 15,comprising a portion of a pipe, the first ultrasonic transducer beingmounted to the pipe portion at a first location, and the secondultrasonic transducer being mounted to the pipe portion at a secondlocation.

Embodiment 17: A computer readable program code comprising computerreadable instructions for executing the method according to one theembodiments 1 to 9.

Embodiment 18: A computer readable memory, the computer readable memorycomprising the computer readable program code of embodiment 17.

Embodiment 19: An application specific electronic component, which isoperable to execute the method according to one of the embodiments 1 to9.

Embodiment 20: A method for determining whether a test device ismeasuring a flow speed of a fluid in a fluid conduit according to one ofembodiments 1 to 5, comprising:

-   -   providing the fluid conduit with a fluid that has a        pre-determined velocity with respect to the fluid conduit,    -   applying a test impulse signal to a first ultrasonic transducer        of the test device, the first ultrasonic transducer being        mounted to the fluid conduit at a first location,    -   receiving a test response signal of the test impulse signal at a        second ultrasonic transducer of the test device, the second        ultrasonic transducer being mounted to the fluid conduit at a        second location,    -   deriving a test measuring signal from the response signal, the        derivation of the test measuring signal comprising reversing the        signal with respect to time,    -   comparing the test measuring signal with a measuring signal that        is emitted at the other one of the first and the second        ultrasonic transducer,    -   wherein the test device is using a method to determine a flow        speed of a fluid in a fluid conduit according to one of        embodiments 1 to 5, if the test measuring signal and the        measuring signal are similar.

Embodiment 21: A device for measuring a flow speed in a travel timeultrasonic flow meter, comprising

-   -   a first connector for a first ultrasonic element,    -   a second connector for a second ultrasonic element,    -   a transmitting unit for sending an impulse signal to the first        connector,    -   a receiving unit for receiving a response signal to the impulse        signal from the second connector,    -   an inverting unit for inverting the selected portion of the        response signal with respect to time to obtain an inverted        signal,    -   a processing unit for deriving a measuring signal from the        inverted signal and storing the measuring signal in the computer        readable memory,    -   wherein using the device for determining a flow speed of a fluid        in a fluid conduit by:    -   providing the fluid conduit with a fluid that has a velocity        with respect to the fluid conduit,    -   applying a measuring signal to one of the first and the second        ultrasonic elements,    -   measuring a first response signal of the measuring signal at the        other one of the first and the second ultrasonic elements,    -   deriving a flow speed of the fluid from the first response        signal, wherein when applying a test impulse signal to a first        ultrasonic element of the test device,    -   receiving a test response signal of the test impulse signal at a        second ultrasonic element of the test device, the second        ultrasonic element being mounted to the fluid conduit at a        second location,    -   deriving a test measuring signal from the response signal, the        derivation of the test measuring signal comprising reversing the        signal with respect to time,    -   wherein the test measuring signal and a measuring signal that is        emitted at the first or the second ultrasonic element are        similar.

Reference 10 flow meter arrangement 11 upstream piezoelectric element 12Pipe 13 downstream piezoelectric element 14 direction of average flow 15first computation unit 16 second computation unit 17 signal path 20signal path 22 piezoelectric element 23 piezoelectric element 31-52piezoelectric elements 60, 60′ flow measurement device 61 firstconnector 62 second connector 63 multiplexer 64 DAC 65 ADC 66demultiplexer 67 signal selection unit 68 signal inversion unit 69bandpass filter 70 memory 71 velocity computation unit 72 impulse signalgenerator 73 measuring signal generator 74 command line 75 command line76 DDS 77 reference oscillator 78 frequency controller register 79numerically controlled oscillator 80 low pass filter

What is claimed is:
 1. A method for determining a flow speed of a fluidin a fluid conduit with a travel time ultrasonic flow meter, the methodcomprising: providing the fluid conduit with a fluid that has apredetermined velocity with respect to the fluid conduit, applying animpulse signal to a first ultrasonic transducer, the first ultrasonictransducer being mounted to the fluid conduit at a first location,receiving a response signal of the impulse signal at a second ultrasonictransducer, the second ultrasonic transducer being located at the fluidconduit at a second location provided upstream or downstream of thefirst ultrasonic transducer, deriving a measuring signal from theresponse signal, the derivation of the measuring signal comprisingselecting a signal portion of the response signal or of a signal derivedtherefrom and reversing the signal portion with respect to time, therebyobtaining an inverted version of the response signal with respect totime, the selected signal portion comprising a first portion around amaximum amplitude of the response signal and a trailing signal portion,the trailing signal portion extending in time behind the arrival time ofthe maximum amplitude, storing the measuring signal for later use,applying the measuring signal to one of the first and the secondultrasonic transducers, the measuring signal comprising a reversedsignal portion with respect to time of the response signal of theimpulse signal or of a signal derived therefrom, measuring a firstresponse signal of the measuring signal at the other one of the firstand the second ultrasonic transducer, deriving a time of flight from thefirst response signal, and deriving a flow speed of the fluid from thetime of flight.
 2. The method of claim 1 further comprising: repeatingthe steps of applying the measuring signal and measuring the responsesignal in the reverse direction to obtain a second response signal,deriving a flow speed of the fluid from the first response signal andthe second response signal.
 3. The method of claim 1 further comprising:repeating the steps of applying an impulse signal and receiving acorresponding response signal multiple times, thereby obtaining aplurality of response signals, deriving the measuring signal from anaverage of the received response signals.
 4. The method of claim 1further comprising increasing the bit-resolution of the digitized signalfor increasing an amplitude of a response signal to the measuringsignal.
 5. The method of claim 1 further comprising decreasing thebit-resolution of the digitized signal for increasing an amplitude of aresponse signal to the measuring signal.
 6. The method of claim 1wherein the bit resolution of the digitized signal with respect to theamplitude is a low bit resolution.
 7. The method of claim 1 furthercomprising processing of at least one of the response signals fordetermining a change in the wall thickness of the conduit or fordetermining material characteristics of the conduit walls by determininglongitudinal and transversal sound wave characteristics.
 8. A device formeasuring a flow speed in a travel time ultrasonic flow meter furthercomprising: a first connector for a first ultrasonic element, a secondconnector for a second ultrasonic element, a transmitting unit forsending an impulse signal to the first connector, a computer readablememory for storing a measuring signal, a receiving unit for receiving aresponse signal to the impulse signal from the second connector, aselection unit for selecting a portion of the received response signalor a signal derived therefrom, an inverting unit for inverting theresponse signal with respect to time to obtain an inverted signal,wherein the inverting unit is provided for inverting the selectedportion of the response signal with respect to time to obtain theinverted signal, one or more processing unit for deriving a measuringsignal from the inverted signal and storing the measuring signal, ameasuring signal generator, the measuring signal generator beingconnectable to the first connector or to the second connector, atransmitting means for sending the measuring signal to the firstconnector, and a receiving unit for receiving a response signal of themeasuring signal from the second connector, wherein the one or moreprocessing unit further derives a time of flight from the receivedresponse signal and a flow speed from the time of flight.
 9. The deviceof claim 8 further comprising a direct digital signal synthesizer, thedirect digital signal synthesizer further comprising the A/D converter,a frequency control register, a reference oscillator, a numericallycontrolled oscillator and a reconstruction low pass filter, the A/Dconverter being connectable to the first and the second connector overthe reconstruction low pass filter.
 10. The device of claim 8 furthercomprising: a first ultrasonic transducer, the first ultrasonictransducer being connected to the first connector, and a secondultrasonic transducer, the second ultrasonic transducer being connectedto the second connector.
 11. The device of claim 8 further comprising aportion of a pipe, the first ultrasonic transducer being mounted to thepipe portion at a first location, and the second ultrasonic transducerbeing mounted to the pipe portion at a second location.
 12. A computerreadable program code comprising computer readable instructions forexecuting the method of claim
 1. 13. A computer readable memory, thecomputer readable memory comprising the computer readable program codeof claim
 12. 14. An application specific electronic component, which isoperable to execute the method of claim
 1. 15. A device for measuring aflow speed in a travel time ultrasonic flow meter comprising: a firstconnector for a first ultrasonic element, a second connector for asecond ultrasonic element, a transmitting unit for sending an impulsesignal to the first connector, a receiving unit for receiving a responsesignal to the impulse signal from the second connector, an invertingunit for inverting the response signal with respect to time to obtain aninverted signal, the inverting compromising reversing an order ofrecorded samples of the received response signal, a processing unit forderiving a measuring signal from the inverted signal and storing themeasuring signal, a selection unit for selecting a portion of a receivedresponse signal or a signal derived therefrom, a measuring signalgenerator for generating a measuring signal comprising a reversed signalportion with respect to time of a response signal of an impulse signalor of a signal derived therefrom, the measuring signal generator beingconnectable to the first connector or to the second connector, a D/Aconverter, the D/A converter being connected to the first connector, anA/D converter, the A/D converter being connected to the secondconnector, a computer readable memory for storing the measuring signal,a transmitting means for sending the measuring signal to the firstconnector, a receiving unit for receiving a response signal of themeasuring signal from the second connector, a processing unit forderiving a flow speed from the received response signal, and a directdigital signal synthesizer, the direct digital signal synthesizercomprising the A/D converter, a frequency control register, a referenceoscillator, a numerically controlled oscillator and a reconstruction lowpass filter, the A/D converter being connectable to the first and thesecond connector over the reconstruction low pass filter.
 16. The deviceof claim 15 further comprising: a first ultrasonic transducer, the firstultrasonic transducer being connected to the first connector, and asecond ultrasonic transducer, the second ultrasonic transducer beingconnected to the second connector.
 17. The device of claim 15 furthercomprising a portion of a pipe, the first ultrasonic transducer beingmounted to the pipe portion at a first location, and the secondultrasonic transducer being mounted to the pipe portion at a secondlocation.